Vehicles today are generally equipped with a steering system which allows a vehicle operator to steer vehicle wheels, and thereby the vehicle, by turning a steering wheel. Many vehicles, such as cars, are also provided with a steering assist system. Such a system may be referred to as power steering, power assisted steering (PAS) or electrical power assisted steering (EPAS). The steering assist system helps the vehicle operator in steering maneuvers by augmenting a steering effort of the steering wheel. Hydraulic or electric actuators thus add energy to the steering mechanism so the vehicle operator needs to provide less effort regardless of conditions.
An EPAS comprises an electric arrangement which provides an overlay torque to the steering system along with the torque the vehicle operator applies on the steering wheel. The overlay torque is usually strong enough to move a steering rack and turn wheels of the host vehicle without any steering wheel input from the vehicle operator. Therefore such an EPAS may be used for controlling steering also in autonomous vehicles. During autonomous driving the EPAS controls the desired front wheels steering angles without any steering wheel input from the vehicle operator.
If a steering actuator fault occurs when the host vehicle is operated autonomously, in the sense that the actuator is not capable of support steering actuation, the vehicle will not be controllable. One fault that could occur is a fault where the overlay torque will be zero, i.e., no actuation occurs at all. As a result, the vehicle will most likely not follow a road curvature and hence will depart from the road. Even if the vehicle operator notices the absence of steering support, it will in many cases take too long time for him/her to regain control in order to be able to avoid the road departure.
To fulfill system safety requirements redundant steering systems, i.e., double steering actuators may be used. However, such systems add complexity and cost to the steering system.
In US20130030651A1 differential braking is used to steer a vehicle such that an accident can be avoided. The collision avoidance system suggested in US20130030651A1 may be useful in some situations, but there remains a need for an improved and/or alternative secondary steering system unit.